As of 18.0.461 there is a new scheduler API callback `applicationBin` which should be implemented by custom schedulers.

applicationBin(self, name, work_item) → str
This callback is used when a node is creating a command that uses an application that can be parameterized by the scheduler. For example there may be UI to control which ‘python’ application should be used for python-based jobs.

Custom scheduler bindings can use their own application ‘names’ to work with custom nodes.

At minimum ‘hython’ and ‘python’ should be supported, as this will eventually be preferred to using the command tokens __PDG_PYTHON__, __PDG_HYTHON__.